Domestic sewage is generally treated by conventional activated sludge process and chemically enhanced primary treatment (CEPT) process. CEPT is an advanced primary treatment process in which inorganic coagulants and organic polymers are added to the sewage stream to enhance the settling of the suspended solids. The process typically includes coagulation of suspended solids by rapid mixing of ferric chloride or polyaluminum chloride followed flocculation of suspended solids, and then sedimentation of the flocs. Owing to the simple operation process and rapid treatment of large quantity sewage in short period of time, CEPT process has been implemented as an alternative treatment process to the conventional process. CEPT is believed to be a cost-effective method for sewage treatment as it has some advantages in sewage treatment, small investment and efficient removal of some pollutants such as phosphate, low energy requirement, and easy to operate and maintain rapid treatment of large quantity of wastewater. CEPT process is particularly very useful for rapidly growing mega cities and developing countries. One of the world's biggest CEPT plant is located at Stonecutter Island, Hong Kong, which is serving a population of around 3 millions and treatment capacity of 1.4 million m3 sewage effluents per day.
Sewage is characterized by the presence of micro-colloidal or suspended particles with a certain amount of surface charge, which remain dispersed for a long time without sinking. However, reduction or elimination of the colloidal surface charge to achieve the critical potential decreases the electrostatic repulsion between small colloids generating particle coalescence, which ultimately results in destabilization and precipitation of colloids.
Chemical coagulants enhance the aggregation and adsorption of dissolved organic colloidal contaminants to form large aggregates, rapid-setting flocs either through charge neutralization or chain-bridging mechanisms which can then be removed by sedimentation and filtration. To enhance the removal efficiency of suspended particles, chemical coagulants such as alum, ferric chloride, ferric sulfate, and lime are usually used. Their major drawbacks are that the coagulant species form rapidly during dilution, and the formation of hydrolysis species cannot be controlled. In addition, CEPT process results in the increase in total dissolved solids (TDS) and production of excessive sludge volumes.
The major issue in sewage treatment is disposal of the large quantity of sludge that generated during sewage treatment process. CEPT process generates the sludge water content greater than 95%. Dewatering of the sludge is an essential that can be achieved by mechanical dewatering process through filter pressing or centrifugation after appropriate preconditioning of sludge. Due to the high strength of suspended particles with high negative charge, the CEPT sludge is difficult to dewater and it requires effective flocculants to improve the pre-dewatering flocculation of sludge flocs. The inorganic metal flocculants and organic polymers are generally used as pre-dewatering flocculants which are expensive. In sludge dewatering process, the typical doses of polyelectrolyte polymers between 1-10 kg per metric ton of dry sludge solids are used, and the polymers thus accounts for 0.1%-1% of the total solid mass of a conditioned sludge. Requirement of huge quantities of the expensive flocculants itself covers about 50% cost of the sludge treatment process.
Synthetic organic polymers and their derivatives pose a number of ecological hazards as some of the derivatives are recalcitrant and their intermediate products are hazardous to human health as their monomer is neurotoxic and carcinogenic. In CEPT process, the organic polymer flocculants are used in two different stages; anionic polymer for sewage flocculation and cationic polymer for sludge flocculation. Some cationic organic polymers used to enhance thickening and dehydration processes could result in strong odors due to their degradation during the storage of dewatered sludge. Therefore, in order to reduce the risks posed by these chemical coagulants, more environment-friendly and safe coagulants for pollutant removal in water treatment with more simplified process remain necessary to develop.
To alleviate concern over the use of synthetic flocculants, currently, microbial flocculants are drawing increasing attention due to their satisfied flocculating effectiveness and safe biodegradable nature. Their biodegradability, harmlessness and lack of secondary pollution, have gained much wider attention and research to date. Some of the microbial flocculants have advantages over other types of flocculants and can be produced economically in large scale. Several research works have provided a demonstration of microbial flocculants in removing suspended solids, chemical oxygen demand (COD), humic acids and heavy metals from several wastewaters, including domestic, brewage and pharmaceutical wastewaters. However, most of these bioflocculants reported to-date are polysaccharides based which are mainly involved in flocculation process via bridging mechanism only. On the other hand, the nature of BioFerric flocculant is a composite of inorganic and organic substances, which could facilitate both coagulation and flocculation of sewage suspended particles. In addition to removal of suspended solids, BioFerric flocculant also removes the nutrients from the sewage.
In view of above issues, there is an unmet need to have a novel BioFerric flocculant that could potentially replace the inorganic and synthetic polymeric flocculants, and reduce the number of steps in the conventional CEPT process and thus reduce the operation cost.